let info = document.getElementById('info');
function checkLoginUser() {
    let userNameInput = document.getElementById('username');
    let userNameInputValue = userNameInput.value;
    let passwordInput = document.getElementById('password');
    let passwordInputValue = passwordInput.value;
    if ('' == userNameInputValue) {
        alert('未填写用户名!')
        userNameInput.focus();
        return false;
    }
    if ('' == passwordInputValue) {
        alert('未填写密码!')
        passwordInput.focus();
        return false;
    }
    return true;
}